The present invention relates to an anti-lock brake system having traction slip control which includes a pedal-actuated, preferably auxiliary-force-assisted braking pressure generator having a master cylinder, to which the wheel brakes are connected by way of main brake lines, auxiliary-pressure hydraulic pumps, and wheel sensors and electronic circuits for determining the wheel rotational behavior and for generating electric braking-pressure control signals which, for the purpose of slip control, serve to control electromagnetically actuatable pressure-fluid inlet valves and outlet valves provided in the pressure-fluid lines. The pistons of the master cylinder are provided with central control valves which, in the brake's release position, open pressure-fluid connections between the pressure-fluid supply reservoir and the pressure chambers and which, in the braking position, close these pressure-fluid connections. The electric motor driven pumps are connected to the supply reservoir, on the one hand, and to the main brake lines, on the other hand.
In known brake systems of this type such as U.S. Pat. No. 4,415,210 (German published patent application No. 30 40 561) and U.S. Pat. No. 4,416,491 (German published patent application No. 30 40 562), a master cylinder having a hydraulic brake power booster connected upstream thereof is used as a braking pressure generator. The auxiliary-pressure supply system includes a hydraulic pump and a hydraulic accumulator from which auxiliary pressure proportional to pedal force is delivered on brake application with the aid of a control valve. On the one hand, this dynamic pressure is transmitted by way of the master cylinder into the static brake circuits connected to the master cylinder. On the other hand, the wheel brakes of one axle, preferably those of the rear axle, are in direct communication with the pressure chamber of the booster into which the pressure proportional to pedal force is introduced through the control valve. For the purpose of slip control, inlet valves are provided in both the static circuits and the dynamic circuit, which valves normally assume their opened position and which, in the event of an imminent locked condition of a wheel, serve to shut off the pressure-fluid flow to the wheel brake effected.
There are also provided outlet valves which allow pressure fluid to discharge from the effected wheel brake to the pressure-compensating reservoir. On commencement of slip control, the booster chamber into which the controlled pressure from the auxiliary-pressure supply system is introduced is connected by way of a main valve with the static brake circuits of the master cylinder in order to replenish the quantity of pressure fluid which is removed from the static circuit through the outlet valves. In addition, for safety reasons, the piston of the master cylinder, or pistons in the case of a tandem master cylinder are reset or fixed by means of a positioning device. The structural complexity required for generating, storing and controlling the hydraulic auxiliary pressure, for dynamic fluid delivery into the static circuits and for safeguarding the brake functions on failure of individual circuits is considerable.
In brake systems of this type, the control signals for the inlet valves and outlet valves are generated by means of electronic circuits, the inputs of which are connected to wheel sensors, for example inductive pickups, for measuring wheel rotational data, and which are able to react to changes in the wheel rotational behavior indicative of an imminent locked condition. The control signals operate to maintain the pressure at the wheel concerned constant, by reducing and increasing the pressure as required.